Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.
The turbine stage of a conventional turbocharger comprises: a turbine housing defining a turbine chamber within which the turbine wheel is mounted; an annular inlet passage defined in the housing between facing radially extending walls arranged around the turbine chamber; an inlet arranged around the inlet passage; and an outlet passage extending from the turbine chamber. The passages and chamber communicate such that pressurised exhaust gas admitted to the inlet flows through the inlet passage to the outlet passage via the turbine chamber and rotates the turbine wheel. It is known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passage so as to deflect gas flowing through the inlet passage towards the direction of rotation of the turbine wheel.
Turbines of this kind may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passage can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied in line with varying engine demands.
In one known type of variable geometry turbine, an axially moveable wall member defines one wall of the inlet passage. The position of the movable wall member relative to a fixed facing wall of the inlet passage is adjustable to control the axial width of the inlet passage. Thus, for example, as exhaust gas flow through the turbine decreases, the inlet passage width may be decreased to maintain the gas velocity and optimise turbine output.
The axially movable wall member may be a “nozzle ring” that is provided with vanes that extend into the inlet passage and through orifices provided in a shroud plate defining the fixed facing wall of the inlet passage, the orifices being designed to accommodate movement of the nozzle ring relative to the shroud. Typically the nozzle ring may comprise a radially extending wall (defining one wall of the inlet passage) and radially inner and outer axially extending walls or flanges that extend into an annular cavity behind the radial face of the nozzle ring. The cavity is formed in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing) and accommodates axial movement of the nozzle ring. The flanges may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. In one common arrangement the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator, which axially displaces the rods.
In an alternative type of variable geometry turbocharger, the nozzle ring is fixed and has vanes that extend from a fixed wall through orifices provided in a moving shroud plate.
Actuators for moving the nozzle ring or movable shroud plate can take a variety of forms, including pneumatic, hydraulic and electric and can be linked to the nozzle ring or shroud plate in a variety of ways. The actuator will generally adjust the position of the nozzle ring or movable shroud plate under the control of an engine control unit (ECU) in order to modify the airflow through the turbine to meet performance requirements.
In addition to the conventional control of a variable geometry turbocharger in an engine fired mode (in which fuel is supplied to the engine for combustion) to optimise gas flow, it is also known to take advantage of the facility to minimise the turbocharger inlet area to provide an engine braking function in an engine braking mode (in which no fuel is supplied for combustion) in which the inlet passage is reduced to smaller areas compared to those in a normal engine fired mode operating range.
Engine brake systems of various forms are widely fitted to vehicle engine systems, in particular to compression ignition engines (diesel engines) used to power large vehicles such as trucks. The engine brake systems may be employed to enhance the effect of the conventional friction brakes acting on the vehicle wheels or, in some circumstances, may be used independently of the normal friction braking system, to control, for example, the downhill speed of a vehicle. With some engine brake systems, the brake is set to activate automatically when the engine throttle is closed (i.e. when the driver lifts his foot from the throttle pedal), and in others the engine brake may require manual activation by the driver, such as depression of a separate brake pedal.
In one form of conventional engine brake system an exhaust valve in the exhaust line is controlled to block partially the engine exhaust when braking is required. This produces an engine braking torque by generating a high backpressure that retards the engine by serving to increase the work done on the engine piston during the exhaust stroke. This braking effect is transmitted to the vehicle wheels through the vehicle drive chain.
With a variable geometry turbine, it is not necessary to provide a separate exhaust valve. Rather, the turbine inlet passage may simply be “closed” to a minimum flow area when braking is required. The level of braking may be modulated by control of the inlet passage size by appropriate control of the axial position of the nozzle ring or movable shroud plate. In a “fully closed” position in an engine braking mode the nozzle ring or movable shroud plate may in some cases about the facing wall of the inlet passage.
A variable geometry turbocharger can also be operated in an engine fired mode so as to close the inlet passage to a minimum width less than the smallest width appropriate for normal engine operating conditions in order to control exhaust gas temperature. The basic principle of operation in such an “exhaust gas heating mode” is to reduce the amount of airflow through the engine for a given fuel supply level (whilst maintaining sufficient airflow for combustion) in order to increase the exhaust gas temperature. This has particular application where a catalytic exhaust after-treatment system is present. In such a system performance is directly related to the temperature of the exhaust gas that passes through it.
To achieve a desirable performance the exhaust gas temperature must be above a threshold temperature (typically lying in a range of about 250° C. to 370° C.) under all engine operating conditions and ambient conditions. Operation of the exhaust gas after-treatment system below the threshold temperature range will cause the system to build up undesirable accumulations.
These must be burnt off in a regeneration cycle to allow the system to return to designed performance levels. In this regard, thermal management or engine regeneration is a pre-determined engine process which uses exhaust gas heating, to close the inlet passage to a minimum width less than the smallest width appropriate for normal engine operating conditions, to heat the exhaust gas to a temperature that will burn off the build-up undesirable accumulations.
In addition, prolonged operation of the exhaust gas after-treatment system below the threshold temperature without regeneration will disable the system and cause the engine to become non-compliant with government exhaust emission regulations.
For the majority of the operating range of, for example, a diesel engine, the exhaust gas temperature will generally be above the required threshold temperature. However, in some conditions, such as light load conditions and/or cold ambient temperature conditions, the exhaust gas temperature can often fall below the threshold temperature. In such conditions the turbocharger can in principle be operated in the exhaust gas heating mode to reduce the turbine inlet passage width with the aim of restricting airflow thereby reducing the airflow cooling effect and increasing exhaust gas temperature.
For both engine braking and exhaust gas heating, it is important to allow some exhaust gas flow through the turbine of the turbocharger. If the exhaust from an engine is restricted to too great an extent, this can lead to excessive heat generation in the engine cylinders, failure of exhaust valves, and the like. There must therefore be provision for at least a minimum leakage flow through the turbine when the nozzle ring or movable shroud plate is moved to a position in which the inlet width is small, or in a fully closed position, for example during engine braking mode or exhaust gas heating mode.
In this respect, due to their high efficiency modern variable geometry turbochargers can generate such high boost pressures even at small inlet widths that their use in an engine braking mode can be problematic as cylinder pressures can approach, or exceed, acceptable limits unless countermeasures are taken (or braking efficiency is sacrificed). Similarly, in relation to exhaust gas heating, the high boost pressures achieved at small inlet widths can actually increase the airflow to the engine, offsetting the effect of the restriction and thus reducing the desired heating effect. By way of example, it is believed that in order to maintain the temperature of exhaust gas in an engine idling at around 1000 rpm, turbine efficiency must be 50% or less.
These problems have been addressed up to a point by EP1435434, which discloses a variable geometry turbine having an annular passageway defined between a radial wall of a moveable wall member and a facing wall of the turbine housing. The moveable wall member is mounted within an annular cavity provided within the housing and has inner and outer annular surfaces. An annular seal is disposed between an annular flange of the moveable wall member and the adjacent inner or outer annular surface of the cavity. The turbine comprises a bypass arrangement in the form of a plurality of radially extending bypass slots provided in the annular flange and distributed in the circumferential direction. Each bypass slot extends through the radial thickness off the annular flange.
The annular seal and bypass passageways move axially relative to one another as the moveable wall member moves. The annular seal and bypass passages are axially located such that as the annular wall member approaches the facing wall of the housing, the bypass passages permit the flow of exhaust gas through the cavity to the turbine wheel, thereby bypassing the annular inlet passageway.
However, with this known bypass arrangement it is only possible to provide this reduction in efficiency at a certain axial position of the movable wall member.
Furthermore, this arrangement provides relatively little control over the amount of the bypass flow that occurs at different axial positions of the movable wall member.
In this regard once the bypass slots (the circumferential array of bypass slots) are moved inboard of the seal, the bypass slots are permanently fully open. Accordingly, any further movement of the bypass slots inboard of the seal, i.e. any further inboard movement of the moveable wall member does not affect the amount of bypass flow.
In addition, this arrangement may not provide a sufficient reduction in efficiency to prevent over-pressurization of the engine cylinders during engine braking, or to off-set the reduction in the cooling effect of the airflow during exhaust gas heating.
Furthermore, the above bypass arrangement can be prone to clogging due to the build-up of soot or other particulate matter, which can reduce the effectiveness of the bypass passages, or even render them inoperable.